1. Field of the Invention
The present invention relates to zoom lens systems and image pickup apparatuses equipped with such systems.
2. Description of the Related Art
Compact, high-resolution zoom lens systems having a wide angle of view to be used as photographic optical systems in image pickup apparatuses that apply solid-state image sensors have been, in recent years, in great demand. Such image pickup apparatuses can be, for example, a video camera, a digital still camera, a broadcast camera, or a silver film camera, or other image pickup apparatus as known by one of ordinary skill in the relevant art or equivalents. For example, in video cameras, the demand for recording high-definition still images in addition to moving pictures is growing, and therefore, a compact lens system with high optical performance is in great demand.
To meet these demands, there are conventional rear-focusing zoom lens systems (for example, see Japanese Patent Laid-Open Nos. 11-305124 (corresponding to U.S. Pat. No. 6,166,864), 8-5913 (corresponding to U.S. Pat. No. 5,847,882), and 2000-267005). Such systems perform focusing by moving a lens unit other than a first lens unit disposed most proximate to an object.
Generally, in comparison to zoom lens systems that perform focusing by moving the first lens unit, the first lens unit in a rear-focusing zoom lens system has a smaller effective diameter. For this reason, the entire lens system is readily reduced in size. Moreover, rear-focusing zoom lens systems are more suitable for close-up photography and macro-photography. Furthermore, rear-focusing zoom lens systems can require only a small amount of driving force for moving the lens unit for focusing since the lens unit can be small and lightweight. Therefore, a focusing operation in rear-focusing zoom lens systems can be performed very quickly.
According to the zoom lens systems discussed in Japanese Patent Laid-Open Nos. 11-305124 and 8-5913, the degree of axial chromatic aberration (longitudinal chromatic aberration) and transverse chromatic aberration (lateral chromatic aberration) is large especially at the telephoto end. For this reason, if these zoom lens systems are to be used in apparatuses that require high resolution and high image quality, such as a digital still camera, these chromatic aberrations become visible.
On the other hand, according to the zoom lens system discussed in Japanese Patent Laid-Open No. 2000-267005, these chromatic aberrations are properly corrected or the error reduced by providing the first lens unit with a lens composed of extraordinary dispersion glass.
Recently, techniques for reducing chromatic aberration in an optical system have been discussed where reduction can be achieved by providing a diffractive optical element in the optical system (for example, see Japanese Patent Laid-Open No. 6-324262 (corresponding to U.S. Pat. No. 5,790,321), U.S. Pat. No. 5,268,790, Japanese Patent Laid-Open No. 11-52238 (corresponding to U.S. Pat. No. 6,606,200), and Japanese Patent Laid-Open No. 11-305126 (corresponding to US AA2003076591)).
If a diffractive optical element is to be used in a photographic system (optical system), a sufficient diffraction efficiency can be attained over the entire visible range. Generally, with only a single-layer diffraction grating, the diffraction efficiency is lowered at wavelengths other than the design wavelength, thus producing undesired diffraction rays of orders other than the design order. This can induce color flare. In view of this, Japanese Patent Laid-Open No. 9-127322 (corresponding to U.S. Pat. No. 6,157,488), for example, discusses a diffractive optical element that includes a plurality of diffraction gratings. Specifically, a material of each diffraction grating and the thickness of each diffraction grating are optimally selected so that a diffractive optical element with high diffraction efficiency over the entire visible range is achieved.
Generally, by increasing the refractive power of each lens unit of a zoom lens system, the entire lens system can be reduced in size and be given a high zoom ratio due to a reduced moving amount of each lens unit for a zooming operation. However, it can be difficult to attain high optical performance since the displacement amount of each aberration, especially the chromatic aberration, increases during the zooming or focusing operation.
For example, in a zoom lens system having a high zoom ratio of 10× or more, if a diffractive optical element is incorporated into the first lens unit to correct the chromatic aberration, there are cases where the angle of light incident on the diffractive optical element changes significantly in response to a change in the angle of view or the focal length. Therefore, it can be necessary in some circumstances to consider the appropriate positioning of the diffractive optical element in order to reduce undesired diffraction rays.
Furthermore, if the chromatic aberration of the entire lens system is to be corrected or reduced solely with a diffractive optical element, it can be difficult to correct the chromatic aberration over the entire zoom range to achieve high quality images while still attaining a high zoom ratio.
Therefore, in order to achieve high optical performance by having the ability to properly correct the chromatic aberration over the entire zoom range from the wide-angle end to the telephoto end while attaining a high zoom ratio, it can be useful to select the appropriate position of the diffractive optical element in the optical system and appropriate materials of optical elements included in the optical system.